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When physicist David Reitze stepped to the podium, smiling, he offered no preamble.

“Ladies and gentleman,” he said. “We have detected gravitational waves.”

His next words — “we did it” — were clipped by the jubilant reaction at the overflowing press conference in Washington D.C. where Reitze, executive director of the Laser Interferometer Gravitational-Wave Observatory (LIGO), spoke alongside his longtime scientific collaborators. Applause swept through a packed lecture room inside the University of Toronto’s Canadian Institute for Theoretical Astrophysics, a reaction that was doubtless matched by groups of researchers worldwide.

The thrill of confirmation seemed barely dented by widespread rumours ahead of Thursday’s announcement: scientists have obtained the first direct evidence of an elusive cosmic feature predicted by Albert Einstein nearly a century ago. Even more excitingly for scientists, the detection of gravitational waves cracks open a whole new way of studying the universe.

Gravitational waves are ripples in the fabric of space-time predicted by Einstein’s general theory of relativity, which celebrated its 100th anniversary late last year. Einstein theorized that powerful enough events — like the merging of two black holes — would send such ripples sweeping across the universe.

A scientist works on a mirror of the entrance telescope of the interferometer Virgo detector in Pisa. Virgo is an instrument created by an international collaboration of labotories used for the detection of gravitational waves. (FRESILLON CYRIL / AFP/GETTY IMAGES)

The Laser Interferometer Gravitational-wave Observatory (LIGO) Livingston Laboratory detector site near Livingston, La., one of two locations that detected a gravitational wave passing through the earth milliseconds apart. (LIGO Laboratory / REUTERS)

Indirect evidence for gravitational waves was gleaned from observing binary pulsars — spinning neutron stars — in the 1970s. But scientists yearned for direct evidence, and Thursday’s announcement is the result of a decades-long hunt to capture it.

At an eventual cost of $1.1 billion (U.S.), one of the largest investments ever made by the U.S. National Science Foundation, the LIGO collaboration built two twin “interferometer” detectors in Hanford, Wash., and Livingston, La.

Inside each interferometer, laser beams are split down two four-kilometre-long arms and bounced off a series of carefully protected mirrors. The beams should arrive back at their source at the same time. A mismatch in their arrival may indicate that a gravitational wave has subtly deformed them as it imperceptibly stretches space-time around us.

But the discrepancy LIGO’s scientists were trying to measure is smaller than the width of a proton, and prosaic local disturbances — a tree falling, a rumbling truck — could disrupt the lasers too. The observatory’s first data run found nothing. The detectors shut down in 2010 for a five-year, $205 million upgrade, boosting their sensitivity.

On Sept. 14, 2015, the detectors had been up and running again for just two days when the two locations each recorded a signal seven milliseconds apart. The signal was so clear the collaborators could see it without even cleaning up the data, and the fact both interferometers recorded it nearly simultaneously strongly indicated it was a true gravitational wave.

Computer simulations showing what a gravitational wave produced by a black-hole merger would look like — including contributions from Canadian Institute for Advanced Research fellow Harald Pfeiffer and other collaborators at CITA, and powered in part by a supercomputer in Vaughan called SciNet — aligned neatly with the signal.

Months of painstaking data analysis confirmed the detection: the laser beams had been deformed by a gravitational wave generated by the merging of two massive black holes 1.3 billion years ago. The peak of the merger caused a characteristic chirp, a ringing that echoed across the universe and passed through our planet just weeks before the centennial of Einstein’s famous theory.

What a gravitational wave sounds like

“We can hear gravitational waves. We can hear the universe. That’s one of the beautiful things about this: we are not only going to be seeing the universe, we are going to be listening to it,” said Gabriela Gonzalez, a physicist and spokesperson for LIGO.

The team is still digging through the data from LIGO’s most recent run. The observatory will reach even greater sensitivities in the coming years, and will be joined by other detectors in Japan, Italy, and India. In December, scientists at the European Space Agency launched the test-run of a space-borne gravitational wave detector, LISA Pathfinder.

“It’s been a very long road, but this is just the beginning,” said Gonzalez.

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